A nucleostemin family GTPase, NS3, acts in serotonergic neurons to regulate insulin signaling and control body size

Convergence of the insulin and serotonin programs in the pancreatic β-cell

OBJECTIVE

Despite their origins in different germ layers, pancreatic islet cells share many common developmental features with neurons, especially serotonin-producing neurons in the hindbrain. Therefore, we tested whether these developmental parallels have functional consequences.

RESEARCH DESIGN AND METHODS

We used transcriptional profiling, immunohistochemistry, DNA-binding analyses, and mouse genetic models to assess the expression and function of key serotonergic genes in the pancreas.

RESULTS

We found that islet cells expressed the genes encoding all of the products necessary for synthesizing, packaging, and secreting serotonin, including both isoforms of the serotonin synthetic enzyme tryptophan hydroxylase and the archetypal serotonergic transcription factor Pet1. As in serotonergic neurons, Pet1 expression in islets required homeodomain transcription factor Nkx2.2 but not Nkx6.1. In β-cells, Pet1 bound to the serotonergic genes but also to a conserved insulin gene regulatory element. Mice lacking Pet1 displayed reduced insulin production and secretion and impaired glucose tolerance.

CONCLUSIONS

These studies demonstrate that a common transcriptional cascade drives the differentiation of β-cells and serotonergic neurons and imparts the shared ability to produce serotonin. The interrelated biology of these two cell types has important implications for the pathology and treatment of diabetes.

Distribution of serotonin (5-HT) and its receptors in the insect brain with focus on the mushroom bodies: lessons from Drosophila melanogaster and Apis mellifera

The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) plays a key role in regulating and modulating various physiological and behavioral processes in both protostomes and deuterostomes. The specific functions of serotonin are mediated by its binding to and subsequent activation of membrane receptors. The vast majority of these receptors belong to the superfamily of G-protein-coupled receptors. We report here the in vivo expression pattern of a recently characterized 5-HT(1) receptor of the honeybee Apis mellifera (Am5-HT(1A)) in the mushroom bodies. In addition, we summarize current knowledge on the distribution of serotonin and serotonin receptor subtypes in the brain and specifically in the mushroom bodies of the fruit fly Drosophila melanogaster and the honeybee. Functional studies in these two species have shown that serotonergic signaling participates in various behaviors including aggression, sleep, circadian rhythms, responses to visual stimuli, and associative learning. The molecular, pharmacological, and functional properties of identified 5-HT receptor subtypes from A. mellifera and D. melanogaster will also be summarized in this review.

Drosophila GTPase nucleostemin 2 changes cellular distribution during larval development and the GTP-binding motif is essential to nucleoplasmic localization

Nucleostemin (NS), a nucleolar guanosine triphosphate (GTP)-binding protein, plays significant roles in cell cycle progression and ribosomal biogenesis. Drosophila Nucleostemin 2 (NS2), a member of the Drosophila NS family, regulates early eye development and is essential to cell survival in vivo, but the underlying mechanisms have yet to be clarified. Biochemical analysis using the recombinant NS2 protein indicated that NS2 has GTPase activity. Immunohistochemistry revealed that NS2 changes in subcellular locus from the nucleolus to the nucleoplasm during larval development, and that a mutation in the ATP/GTP-binding site motif A (p-loop) prevents nuclear localization of NS2 and results in cytoplasmic distribution. Furthermore, downregulation of NS2 altered the rRNA proportions between the nucleus and the cytoplasm. These results suggest that NS2 at least requires GTP to import into the nucleoplasm.

Insulin-producing cells in the brain of adult Drosophila are regulated by the serotonin 5-HT1A receptor

Insulin signaling regulates lifespan, reproduction, metabolic homeostasis, and resistance to stress in the adult organism. In Drosophila, there are seven insulin-like peptides (DILP1-7). Three of these (DILP2, 3 and 5) are produced in median neurosecretory cells of the brain, designated IPCs. Previous work has suggested that production or release of DILPs in IPCs can be regulated by a factor secreted from the fat body as well as by neuronal GABA or short neuropeptide F. There is also evidence that serotonergic neurons may regulate IPCs. Here, we investigated mechanisms by which serotonin may regulate the IPCs. We show that the IPCs in adult flies express the 5-HT(1A), but not the 5-HT(1B) or 5-HT(7) receptors, and that processes of serotonergic neurons impinge on the IPC branches. Knockdown of 5-HT(1A) in IPCs by targeted RNA interference (RNAi) leads to increased sensitivity to heat, prolonged recovery after cold knockdown and decreased resistance to starvation. Lipid metabolism is also affected, but no effect on growth was seen. Furthermore, we show that DILP2-immunolevels in IPCs increase after 5-HT(1A) knockdown; this is accentuated by starvation. Heterozygous 5-HT(1A) mutant flies display the same phenotype in all assays, as seen after targeted 5-HT(1A) RNAi, and flies fed the 5-HT(1A) antagonist WAY100635 display reduced lifespan at starvation. Our findings suggest that serotonin acts on brain IPCs via the 5-HT(1A) receptor, thereby affecting their activity and probably insulin signaling. Thus, we have identified a second inhibitory pathway regulating IPC activity in the Drosophila brain.

Sensory perception and aging in model systems: from the outside in

Sensory systems provide organisms from bacteria to humans with the ability to interact with the world. Numerous senses have evolved that allow animals to detect and decode cues from sources in both their external and internal environments. Recent advances in understanding the central mechanisms by which the brains of simple organisms evaluate different cues and initiate behavioral decisions, coupled with observations that sensory manipulations are capable of altering organismal lifespan, have opened the door for powerful new research into aging. Although direct links between sensory perception and aging have been established only recently, here we discuss these initial discoveries and evaluate the potential for different forms of sensory processing to modulate lifespan across taxa. Harnessing the neurobiology of simple model systems to study the biological impact of sensory experiences will yield insights into the broad influence of sensory perception in mammals and may help uncover new mechanisms of healthy aging.

Insulin production and signaling in renal tubules of Drosophila is under control of tachykinin-related peptide and regulates stress resistance

The insulin-signaling pathway is evolutionarily conserved in animals and regulates growth, reproduction, metabolic homeostasis, stress resistance and life span. In Drosophila seven insulin-like peptides (DILP1-7) are known, some of which are produced in the brain, others in fat body or intestine. Here we show that DILP5 is expressed in principal cells of the renal tubules of Drosophila and affects survival at stress. Renal (Malpighian) tubules regulate water and ion homeostasis, but also play roles in immune responses and oxidative stress. We investigated the control of DILP5 signaling in the renal tubules by Drosophila tachykinin peptide (DTK) and its receptor DTKR during desiccative, nutritional and oxidative stress. The DILP5 levels in principal cells of the tubules are affected by stress and manipulations of DTKR expression in the same cells. Targeted knockdown of DTKR, DILP5 and the insulin receptor dInR in principal cells or mutation of Dilp5 resulted in increased survival at either stress, whereas over-expression of these components produced the opposite phenotype. Thus, stress seems to induce hormonal release of DTK that acts on the renal tubules to regulate DILP5 signaling. Manipulations of S6 kinase and superoxide dismutase (SOD2) in principal cells also affect survival at stress, suggesting that DILP5 acts locally on tubules, possibly in oxidative stress regulation. Our findings are the first to demonstrate DILP signaling originating in the renal tubules and that this signaling is under control of stress-induced release of peptide hormone.

Human disease models in Drosophila melanogaster and the role of the fly in therapeutic drug discovery

The common fruit fly, Drosophila melanogaster, is a well studied and highly tractable genetic model organism for understanding molecular mechanisms of human diseases. Many basic biological, physiological, and neurological properties are conserved between mammals and D. melanogaster, and nearly 75% of human disease-causing genes are believed to have a functional homolog in the fly. In the discovery process for therapeutics, traditional approaches employ high-throughput screening for small molecules that is based primarily on in vitro cell culture, enzymatic assays, or receptor binding assays. The majority of positive hits identified through these types of in vitro screens, unfortunately, are found to be ineffective and/or toxic in subsequent validation experiments in whole-animal models. New tools and platforms are needed in the discovery arena to overcome these limitations. The incorporation of D. melanogaster into the therapeutic discovery process holds tremendous promise for an enhanced rate of discovery of higher quality leads. D. melanogaster models of human diseases provide several unique features such as powerful genetics, highly conserved disease pathways, and very low comparative costs. The fly can effectively be used for low- to high-throughput drug screens as well as in target discovery. Here, we review the basic biology of the fly and discuss models of human diseases and opportunities for therapeutic discovery for central nervous system disorders, inflammatory disorders, cardiovascular disease, cancer, and diabetes. We also provide information and resources for those interested in pursuing fly models of human disease, as well as those interested in using D. melanogaster in the drug discovery process.

Insulin signaling, lifespan and stress resistance are modulated by metabotropic GABA receptors on insulin producing cells in the brain of Drosophila

Insulin-like peptides (ILPs) regulate growth, reproduction, metabolic homeostasis, life span and stress resistance in worms, flies and mammals. A set of insulin producing cells (IPCs) in the Drosophila brain that express three ILPs (DILP2, 3 and 5) have been the main focus of interest in hormonal DILP signaling. Little is, however, known about factors that regulate DILP production and release by these IPCs. Here we show that the IPCs express the metabotropic GABA_B receptor (GBR), but not the ionotropic GABA_A receptor subunit RDL. Diminishing the GBR expression on these cells by targeted RNA interference abbreviates life span, decreases metabolic stress resistance and alters carbohydrate and lipid metabolism at stress, but not growth in Drosophila. A direct effect of diminishing GBR on IPCs is an increase in DILP immunofluorescence in these cells, an effect that is accentuated at starvation. Knockdown of irk3, possibly part of a G protein-activated inwardly rectifying K⁺ channel that may link to GBRs, phenocopies GBR knockdown in starvation experiments. Our experiments suggest that the GBR is involved in inhibitory control of DILP production and release in adult flies at metabolic stress and that this receptor mediates a GABA signal from brain interneurons that may convey nutritional signals. This is the first demonstration of a neurotransmitter that inhibits insulin signaling in its regulation of metabolism, stress and life span in an invertebrate brain.

Inositol 1,4,5-trisphosphate receptor and dSTIM function in Drosophila insulin-producing neurons regulates systemic intracellular calcium homeostasis and flight

Calcium (Ca(2+)) signaling is known to regulate the development, maintenance and modulation of activity in neuronal circuits that underlie organismal behavior. In Drosophila, intracellular Ca(2+) signaling by the inositol 1,4,5-trisphosphate receptor and the store-operated channel (dOrai) regulates the formation and function of neuronal circuits that control flight. Here, we show that restoring InsP(3)R activity in insulin-producing neurons of flightless InsP(3)R mutants (itpr) during pupal development can rescue systemic flight ability. Expression of the store operated Ca(2+) entry (SOCE) regulator dSTIM in insulin-producing neurons also suppresses compromised flight ability of InsP(3)R mutants suggesting that SOCE can compensate for impaired InsP(3)R function. Despite restricted expression of wild-type InsP(3)R and dSTIM in insulin-producing neurons, a global restoration of SOCE and store Ca(2+) is observed in primary neuronal cultures from the itpr mutant. These results suggest that restoring InsP(3)R-mediated Ca(2+) release and SOCE in a limited subset of neuromodulatory cells can influence systemic behaviors such as flight by regulating intracellular Ca(2+) homeostasis in a large population of neurons through a non-cell-autonomous mechanism.

Molecular mechanisms of metabolic regulation by insulin in Drosophila

The insulin signalling pathway is highly conserved from mammals to Drosophila. Insulin signalling in the fly, as in mammals, regulates a number of physiological functions, including carbohydrate and lipid metabolism, tissue growth and longevity. In the present review, I discuss the molecular mechanisms by which insulin signalling regulates metabolism in Drosophila, comparing and contrasting with the mammalian system. I discuss both the intracellular signalling network, as well as the communication between organs in the fly.

Regulation of tissue growth through nutrient sensing

Nutrition is a key regulator of tissue growth. In animals, nutritional status is monitored and signaled at both the cellular and systemic levels. The main mediator of cellular nutrient sensing is the protein kinase TOR (target of rapamycin). TOR receives information from levels of cellular amino acids and energy, and it regulates the activity of processes involved in cell growth, such as protein synthesis and autophagy. Insulin-like signaling is the main mechanism of systemic nutrient sensing and mediates its growth-regulatory functions largely through the phosphatidylinositol 3-kinase (PI3K)/AKT protein kinase pathway. Other nutrition-regulated hormonal mechanisms contribute to growth control by modulating the activity of insulin-like signaling. The pathways mediating signals from systemic and cellular levels converge, allowing cells to combine information from both sources. Here we give an overview of the mechanisms that adjust animal tissue growth in response to nutrition and highlight some general features of the signaling pathways involved.

Ecdysteroid hormone action

Several reviews devoted to various aspects of ecdysone research have been published during the last few years. Therefore, this article concentrates mainly on the considerable progress in ecdysone research observed recently, and will cover the results obtained during the last 2 years. The main emphasis is put on the molecular mode of ecdysteroid receptor-mediated hormone action. Two examples of interaction with other hormonal signalling pathways are described, namely crosstalk with juvenile hormone and insulin. Some selected, recently investigated examples of the multitude of hormonal responses are described. Finally, ecological aspects and some practical applications are discussed.

Remote control of insulin secretion by fat cells in Drosophila

Insulin-like peptides (ILPs) couple growth, metabolism, longevity, and fertility with changes in nutritional availability. In Drosophila, several ILPs called Dilps are produced by the brain insulin-producing cells (IPCs), from which they are released into the hemolymph and act systemically. We show here that in response to nutrient deprivation, brain Dilps are no longer secreted and accumulate in the IPCs. We further demonstrate that the larval fat body, a functional homolog of vertebrate liver and white fat, couples the level of circulating Dilps with dietary amino acid levels by remotely controlling Dilp release through a TOR/RAPTOR-dependent mechanism. We finally use ex vivo tissue coculture to demonstrate that a humoral signal emitted by the fat body transits through the hemolymph and activates Dilp secretion in the IPCs. Thus, the availability of nutrients is remotely sensed in fat body cells and conveyed to the brain IPCs by a humoral signal controlling ILP release.

Metabolism by remote control

Drosophila melanogaster produce insulin-like peptides in specialized neuroendocrine cells to regulate growth, metabolism, aging, and reproduction. In this issue of Cell Metabolism, Géminard et al. (2009) describe how secretion of insulin-like peptides is remotely controlled by the fat body (an adipose, liver-like tissue) in response to dietary amino acids.

Knockdown of the Drosophila GTPase nucleostemin 1 impairs large ribosomal subunit biogenesis, cell growth, and midgut precursor cell maintenance

Mammalian nucleostemin (NS) is a nucleolar guanosine triphosphate-binding protein implicated in cell cycle progression, stem cell proliferation, and ribosome assembly. Drosophila melanogaster contains a four-member nucleostemin family (NS1-4). NS1 is the closest orthologue to human NS; it shares 33% identity and 67% similarity with human NS. We show that NS1 has intrinsic GTPase and ATPase activity and that it is present within nucleoli of most larval and adult cells. Endogenous NS1 and lightly expressed green fluorescent protein (GFP)-NS1 enrich within the nucleolar granular regions as expected, whereas overexpressed GFP-NS1 localized throughout the nucleolus and nucleoplasm, and to several transcriptionally active interbands of polytene chromosomes. Severe overexpression correlated with the appearance of melanotic tumors and larval/pupal lethality. Depletion of 60% of NS1 transcripts also lead to larval and pupal lethality. NS1 protein depletion>95 correlated with the loss of imaginal island (precursor) cells in the larval midgut and to an apparent block in the nucleolar release of large ribosomal subunits in terminally differentiated larval midgut polyploid cells. Ultrastructural examination of larval Malpighian tubule cells depleted for NS1 showed a loss of cytoplasmic ribosomes and a concomitant appearance of cytoplasmic preautophagosomes and lysosomes. We interpret the appearance of these structures as indicators of cell stress response.

Inositol 1,4,5- trisphosphate receptor function in Drosophila insulin producing cells

The Inositol 1,4,5- trisphosphate receptor (InsP₃R) is an intracellular ligand gated channel that releases calcium from intracellular stores in response to extracellular signals. To identify and understand physiological processes and behavior that depends on the InsP₃ signaling pathway at a systemic level, we are studying Drosophila mutants for the InsP₃R (itpr) gene. Here, we show that growth defects precede larval lethality and both are a consequence of the inability to feed normally. Moreover, restoring InsP₃R function in insulin producing cells (IPCs) in the larval brain rescues the feeding deficit, growth and lethality in the itpr mutants to a significant extent. We have previously demonstrated a critical requirement for InsP₃R activity in neuronal cells, specifically in aminergic interneurons, for larval viability. Processes from the IPCs and aminergic domain are closely apposed in the third instar larval brain with no visible cellular overlap. Ubiquitous depletion of itpr by dsRNA results in feeding deficits leading to larval lethality similar to the itpr mutant phenotype. However, when itpr is depleted specifically in IPCs or aminergic neurons, the larvae are viable. These data support a model where InsP₃R activity in non-overlapping neuronal domains independently rescues larval itpr phenotypes by non-cell autonomous mechanisms.

The transcriptional response of the islet to pregnancy in mice

The inability of the ss-cell to meet the demand for insulin brought about by insulin resistance leads to type 2 diabetes. In adults, ss-cell replication is one of the mechanisms thought to cause the expansion of ss-cell mass. Efforts to treat diabetes require knowledge of the pathways that drive facultative ss-cell proliferation in vivo. A robust physiological stimulus of ss-cell expansion is pregnancy and identifying the mechanisms underlying this stimulus may provide therapeutic leads for the treatment of type 2 diabetes. The peak in ss-cell proliferation during pregnancy occurs on d 14.5 of gestation in mice. Using advanced genomic approaches, we globally characterize the gene expression signature of pancreatic islets on d 14.5 of gestation during pregnancy. We identify a total of 1907 genes as differentially expressed in the islet during pregnancy. The islet's ability to compensate for relative insulin deficiency during metabolic stress is associated with the induction of both proliferative and survival pathways. A comparison of the genes induced in three different models of islet expansion suggests that diverse mechanisms can be recruited to expand islet mass. The identification of many novel genes involved in islet expansion during pregnancy provides an important resource for diabetes researchers to further investigate how these factors contribute to the maintenance of not only islet mass, but ultimately ss-cell mass.

Role of serotonergic neurons in the Drosophila larval response to light

Background

Drosophila larval locomotion consists of forward peristalsis interrupted by episodes of pausing, turning and exploratory behavior (head swinging). This behavior can be regulated by visual input as seen by light-induced increase in pausing, head swinging and direction change as well as reduction of linear speed that characterizes the larval photophobic response. During 3^rd instar stage, Drosophila larvae gradually cease to be repelled by light and are photoneutral by the time they wander in search for a place to undergo metamorphosis. Thus, Drosophila larval photobehavior can be used to study control of locomotion.

Results

We used targeted neuronal silencing to assess the role of candidate neurons in the regulation of larval photobehavior. Inactivation of DOPA decarboxylase (Ddc) neurons increases the response to light throughout larval development, including during the later stages of the 3^rd instar characterized by photoneutral response. Increased response to light is characterized by increase in light-induced direction change and associated pause, and reduction of linear movement. Amongst Ddc neurons, suppression of the activity of corazonergic and serotonergic but not dopaminergic neurons increases the photophobic response observed during 3^rd instar stage. Silencing of serotonergic neurons does not disrupt larval locomotion or the response to mechanical stimuli. Reduced serotonin (5-hydroxytryptamine, 5-HT) signaling within serotonergic neurons recapitulates the results obtained with targeted neuronal silencing. Ablation of serotonergic cells in the ventral nerve cord (VNC) does not affect the larval response to light. Similarly, disruption of serotonergic projections that contact the photoreceptor termini in the brain hemispheres does not impact the larval response to light. Finally, pan-neural over-expression of 5-HT1A_Dro receptors, but not of any other 5-HT receptor subtype, causes a significant decrease in the response to light of 3^rd instar larvae.

Conclusion

Our data demonstrate that activity of serotonergic and corazonergic neurons contribute to the control of larval locomotion by light. We conclude that this control is carried out by 5-HT neurons located in the brain hemispheres, but does not appear to occur at the photoreceptor level and may be mediated by 5-HT1A_Dro receptors. These findings provide new insights into the function of 5-HT neurons in Drosophila larval behavior as well as into the mechanisms underlying regulation of larval response to light.

Nucleostemin: a latecomer with new tricks

Nucleostemin was first identified in neural stem cells and has become a focus of research in cell cycle control, tumorigenesis and cellular senescence. As the biology of nucleostemin begins to be unveiled in multiple species, an ensuing task is to resolve the apparent differences between the functions of mammalian and invertebrate nucleostemin and its homologues, an issue of pressing interest given the role of nucleostemin in stem cell self-renewal and tissue regeneration. A genome-wide search reveals that nucleostemin and its closest homologue, GNL3L, only emerge as separate genes in vertebrates and possess conserved protein sequences as evolution proceeded to the Mammalia. The invertebrate orthologue of nucleostemin and GNL3L resembles GNL3L more than it does nucleostemin in function, raising the idea that nucleostemin acquires new properties while GNL3L inherits an evolutionarily fixed role, and that the birth of nucleostemin may signify the appearance of new functional features in the vertebrate lineage.

Increased insulin/insulin growth factor signaling advances the onset of metamorphosis in Drosophila

Mechanisms by which attainment of specific body sizes trigger developmental transitions to adulthood (e.g. puberty or metamorphosis) are incompletely understood. In Drosophila, metamorphosis is triggered by ecdysone synthesis from the prothoracic gland (PG), whereas growth rate is increased by insulin/insulin growth factor signalling (IIS). Transgene-induced activation of PI3K, the major effector of IIS, within the PG advances the onset of metamorphosis via precocious ecdysone synthesis, raising the possibility that IIS triggers metamorphosis via PI3K activation in the PG. Here we show that blocking the protein kinase A (PKA) pathway in the insulin producing cells (IPCs) increases IIS. This increased IIS increases larval growth rate and also advances the onset of metamorphosis, which is accompanied by precocious ecdysone synthesis and increased transcription of at least one ecdysone biosynthetic gene. Our observations suggest that IIS is regulated by PKA pathway activity in the IPCs. In addition, taken together with previous findings, our observations are consistent with the possibility that, in Drosophila, attainment of a specific body size triggers metamorphosis via the IIS-mediated activation of PI3K and hence ecdysone synthesis in the PG.

Insulin/IGF-like signalling, the central nervous system and aging

Enormous strides in understanding aging have come from the discovery that mutations in single genes can extend healthy life-span in laboratory model organisms such as the yeast Saccharomyces, the fruit fly Drosophila melanogaster, the nematode worm Caenorhabditis elegans and the mouse. IIS [insulin/IGF (insulin-like growth factor)-like signalling] stands out as an important, evolutionarily conserved pathway involved in the determination of lifespan. The pathway has diverse functions in multicellular organisms, and mutations in IIS can affect growth, development, metabolic homoeostasis, fecundity and stress resistance, as well as lifespan. The pleiotropic nature of the pathway and the often negative effects of its disruption mean that the extent, tissue and timing of IIS manipulations are determinants of a positive effect on lifespan. One tissue of particular importance for lifespan extension in diverse organisms is the CNS (central nervous system). Although lowered IIS in the CNS can extend lifespan, IIS is also widely recognized as being neuroprotective and important for growth and survival of neurons. In the present review, we discuss our current understanding of the role of the nervous system in extension of lifespan by altered IIS, and the role of IIS in determination of neuronal function during aging. The nervous system can play both endocrine and cell-autonomous roles in extension of lifespan by IIS, and the effects of IIS on lifespan and neuronal function can be uncoupled to some extent. Tissue-specific manipulation of IIS and the cellular defence mechanisms that it regulates will better define the ways in which IIS affects neuronal and whole-organism function during aging.

Serotonin and insulin signaling team up to control growth in Drosophila

Neuroendocrine signaling pathways play a central role in modulating animal body size in response to environmental signals. Little is known, however, regarding how these neuroendocrine circuits are controlled. An important advance in this area is reported in this issue of Genes & Development by Kaplan and colleagues (pp. 1877-1893), who show that serotonergic neurons regulate the growth of peripheral tissues in Drosophila through the insulin/IGF pathway.